fn: remove flaky test

In this commit, we remove TestPropForEachConcOutperformsMapWhenExpensive
as it's flaky and causes the CI to fail. The previous commit intro-
duced a replacement, which benchmarked the functions.

fn/slice_test.go

@@ -300,48 +300,6 @@ func TestPropForEachConcMapIsomorphism(t *testing.T) {
 	}
 }
 
-// TestPropForEachConcOutperformsMapWhenExpensive ensures the property that
-// ForEachConc will beat Map in a race in circumstances where the computation in
-// the argument closure is expensive.
-func TestPropForEachConcOutperformsMapWhenExpensive(t *testing.T) {
-	f := func(incSize int, s []int) bool {
-		if len(s) < 2 {
-			// Intuitively we don't expect the extra overhead of
-			// ForEachConc to justify itself for list sizes of 1 or
-			// 0.
-			return true
-		}
-
-		inc := func(i int) int {
-			time.Sleep(time.Millisecond)
-			return i + incSize
-		}
-		c := make(chan bool, 1)
-
-		go func() {
-			Map(s, inc)
-			select {
-			case c <- false:
-			default:
-			}
-		}()
-
-		go func() {
-			ForEachConc(s, inc)
-			select {
-			case c <- true:
-			default:
-			}
-		}()
-
-		return <-c
-	}
-
-	if err := quick.Check(f, nil); err != nil {
-		t.Fatal(err)
-	}
-}
-
 func TestPropFindIdxFindIdentity(t *testing.T) {
 	f := func(div, mod uint8, s []uint8) bool {
 		if div == 0 || div == 1 {